Mobile equipment, such as a cellular phone, uses a power amplifier circuit to amplify the power of a radio frequency (RF) signal to be transmitted to a base station. The power amplifier circuit can employ a configuration in which a plurality of unit transistors connected in parallel are arranged on a semiconductor substrate (hereinafter referred to also as “the multi-finger configuration”) in order to satisfy the requirement for a high output power level.
In the foregoing multi-finger configuration, a variation in temperature may take place among the unit transistors, depending on the placement of the unit transistors. Transistors generally have a temperature characteristic in which current passing therethrough increases as the temperatures thereof increase. This poses a problem in that a variation in temperature among a plurality of unit transistors causes more current to pass through unit transistors having relatively high temperatures, thus leading to shortage of output power or deterioration of power efficiency and further to destruction of the transistors due to thermal runaway.
As a solution to the aforesaid problem, Japanese Laid-Open Patent Publication No. 2006-186159, for example, describes a configuration in which a plurality of unit transistors are arranged such that fewer unit transistors are placed on the inner side of a semiconductor substrate than unit transistors placed on the outer side of the semiconductor substrate. According to the configuration, the variation in temperature of the entire substrate is reduced by placing a smaller number of unit transistors on the inner side on the premise that the temperature on the inner side of a semiconductor substrate is the highest.
However, in Japanese Laid-Open Patent Publication No. 2006-186159, the difference in the degree of heat dissipation in a semiconductor substrate is not considered. More specifically, heat does not necessarily stay on the inner side of the semiconductor substrate and, for example, the outer side of the semiconductor substrate is closer to an outer frame, so that the region of heat dissipation is limited. Hence, the degree of heat dissipation may be lower than that on the inner side. In this case, the temperatures of the unit transistors arranged on the outer side of the semiconductor substrate may excessively increase.